The invention relates to the art of vehicle braking systems. The invention finds particular application in heavy trucks, tractors and buses. The invention will be described in relation to an air pressure braking system of a tractor-trailer truck system. However, the invention may be applied to other braking systems such as those of other kinds of vehicles and those based on other technologies such as hydraulic or electronic braking systems.
U.S. Pat. No. 4,844,553 to Bliss and U.S. Pat. No. 4,330,157, incorporated herein by reference, disclose control valve arrangements for use in a braking system. The valve arrangements include a pair of valve actuators or plungers that are operated by a vehicle operator. One of the actuators controls communication between an air pressure source and a tractor parking/emergency brake system. The tractor parking/emergency brake system includes spring brakes that are actuated to resist wheel rotation. A force of an included spring or set of springs in the spring brakes urges a braking member against the wheel. During vehicle operation, air pressure is applied against a piston that is associated with the springs. The force applied against the piston counteracts the force of the springs thereby releasing the spring brake and allowing the wheels of the vehicle to rotate. The other actuator controls communication between the pressure source and service brakes of a trailer.
In addition to providing for manual application and release of the parking/emergency brakes, the disclosed valve arrangements also provide for the automatic application of brakes should the source or actuation pressure fall below a threshold. That is to say, the first actuator is forced into a pressure exhaust position by internal springs if the source pressure drops below a first threshold. Similarly the other actuator is forced into a pressure exhaust position if the pressure drops below a second threshold. This provides for the trailer brakes and the tractor brakes to be applied in an appropriate order should a malfunction or leak occur in the braking system.
One feature the prior art valve arrangements do not provide is an assurance that a vehicle operator has actuated the service brakes before the parking brakes are released. As a result, the vehicle could roll unexpectedly when the operator releases the parking brakes.
Therefore, a braking system interlock arrangement that requires the service brakes of a vehicle to be engaged before the parking brakes of the vehicle can be disengaged is desired.
To that end, a new vehicle braking control system has been developed. The braking control system is operative to prevent the parking brakes from releasing the wheels of the vehicle unless service braking is applied.
A valve assembly includes a double check valve and an inlet valve. The double check valve includes an inlet port, a locking pressure port, and an outlet port. The check valve is operative to place one of the inlet port and the locking pressure port in communication with the outlet port. Additionally, the check valve prevents communication between the inlet port and the locking port. The position of the double check valve is based on relative air pressures at the inlet and locking ports.
The inlet valve includes a second inlet port, a second outlet port, and a control port, the control port being in communication with the outlet port of the double check valve. The inlet valve is operative to place the second inlet port in communication with the second outlet port when a pressure at the control port is above a threshold.
Typically a pressure source is connected to the second inlet port and the second outlet port delivers pressurized air from the second inlet port to the second air pressure sub-system.
The double check valve and inlet valve work together and form a pressure sensor, a system pressure valve, and a latching mechanism. The system pressure valve is controlled by the pressure sensor. For example, the system pressure valve opens when a sensed pressure is above a first threshold and closes when the sensed pressure is below a second threshold. The first and second thresholds may be the same. Preferably, the second threshold is below the first threshold. The system pressure valve includes a system pressure valve inlet and a system pressure valve outlet. The system pressure valve is operative to direct pressurized air delivered to the system pressure valve inlet to the secondary braking system through the system pressure valve outlet when the system pressure valve is open and to prevent pressurized air from reaching the secondary braking system when the system pressure valve is closed. The latching mechanism is operative to direct a higher one of a primary braking system control pressure and a secondary braking system releasing pressure to the pressure sensor. As long as the pressure directed to the pressure sensor is above the second threshold, the system pressure valve is kept open.
One advantage of the present invention is the prevention of unexpected vehicle movements.
Another advantage of the present invention resides in compatibility of the invention with prior braking systems.
Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.